sometimes there is wiring that has no place it might be there for a option your truck did not come with they build all the wiring harness the same and the trucks options difer if you dont remember taking it off chances are it has no place does everything work and it starts if so you should be good

yes - that plug is definitely for your knock sensors. Take off the top timing belt cover and you'll find the cable from both sensors which are located in the valley beneath the intake manifold. You may be able to get from the top with out removing cover. That's why you got your error message for the knock sensors